1. Field of the Invention
The invention relates in general to a planar antenna, and more particularly to a planar antenna utilizing right-handed and left-handed transmission lines.
2. Description of the Related Art
In the wireless and mobile communication applications, the wireless devices, such as mobile phones, wireless network cards, and other mobile computing devices, are desired to be compact in size. One important factor for compactness of the wireless devices is to reduce the size of their antennas.
The basic half-wavelength center-fed dipole and quarter-wavelength monopole or ground plane are still widely used. However, numerous variations have been created to improve performance or adapt to limited physical conditions. One popular antenna is the inverted-F antenna, which can be found on mobile phones, WLAN hardware, and other small wireless devices. The performance is similar to a quarter-wave ground plane. Another example is a patch antenna. The patch antenna spans about one half-wavelength in diameter, which has a slight gain and strong radiation in a direction perpendicular to the patch and is used in some 802.11 wide local-area-network (WLAN) antennas.
The above-mentioned antennas are conventional right-handed transmission line (RH TL), characterized by the delay phase changing. In contrast, a left-handed transmission line (LH TL) is characterized by the advance phase changing. The LH TL occurs due to the study and discussion about metamaterial recently. Specifically left-handed materials (LHMs) possessing negative refractive index have drawn tremendous interests in both scientific and engineering fields. This kind of artificial structure has been utilized for many guided and unguided wave applications. The unique characteristics have been considered to be valuable topics.
Antennas using left-handed transmission lines are proposed by researchers using sophisticated and complex structure for their designed operation characteristic. One kind of practical approaches to synthesize LHMs is based on the backward wave supported by a distributed or equivalent lumped ladder network. With the advantage of backfire-to-endfire scanning, electronically scanned leaky-wave antennas were designed for the desired operation characteristics.
A composite Right/Left-Handed Transmission Line (CRLH TL) has also been developed wherein RH TL and LH TL can be embedded into each other. However, in implementation, the circuit elements of a CRLH TL are valid for very small electrical length θ with the same accuracy as sin θ approaches unity. The zeroth-order resonance (ZOR) makes use of the opposite phase properties of RH and LH TL and has been proved experimentally. The physical size of such antenna can be arbitrary regardless the operation frequency since it is specified by the value of the capacitances and inductances instead of the wavelength. However, it has relatively less compact size including the matching structure.